With development of high performance CPU (Central Processing Unit), switching converter with low output voltage and high output current is needed, and other good performances such as heat performance, EMI performance and fast transient response are also needed. Multiphase switching converter having a plurality of switching circuits is widely used to power high performance CPU. Constant ON-time control is widely employed in switching converter per fast transient response, simple circuit structure and smooth mode transition. A constant ON-time control circuit for controlling a multiphase switching converter usually provides a comparison signal by comparing an output voltage of the multiphase switching converter with a reference signal, and a plurality of switching circuits of the multiphase switching converter are controlled based on the comparison signal. For example, when the output voltage is smaller than the reference signal, the comparison signal becomes high voltage level and a corresponding switching circuit is turned ON. However, for a constant ON-time controlled multiphase switching converter, the output voltage will overshoot during a fast load step-down transient process.